1. Field of the Invention
This invention relates to monolithic analog filter circuits and, more specifically, to a programmable switched-capacitor transversal finite-impulse-response (FIR) filter implemented without active summation to avoid operational amplifier slew-rate limitations on filter bandwidth.
2. Discussion of the Related Art
The direct form of a Finite-Impulse-Response (FIR) nonrecursive filter is the tapped-delay-line filter or transversal filter in which the continuous input signal is sampled at a sampling rate f.sub.s =1/T. The sampling section is followed by M delay stages (FIG. 1), each of which delays the associated sample by the sampling period T. At each node, the delayed signal sample is multiplied by a weighting coefficient and the products are summed to produce a discrete time filter output signal. FIR filters in symmetrical form provide truly linear phase response at frequencies up to the Nyquist frequency of f.sub.s /2, an advantage not available in other filter forms. Moreover, such filters provide very narrow transition bands on the order of 150dB per octave.
High-speed implementation of the analog transversal filter is readily accomplished using a charge-transfer device such as a charge-coupled device (CCD) or a bucket brigade device (BBD). Fully digital implementations are also well-known in the art. Although the fully digital implementations can be programmed by employing digital weighting functions, the faster CCD and BBD transversal filters are not programmable because the established tap weight multipliers may not be altered by program control to provide a different transversal filter characteristic. Because a programmable analog transversal filter is very useful, many practitioners have proposed strategies for providing variable tap weight multipliers in an analog transversal filter.
The fastest FIR filters are perhaps those realized on monolithic integrated circuits (ICs) implemented using CCD or BBD technology. For instance, in U.S. Pat. No. 4,316,258, Jean L. Berger discloses a digitally-programmable filter using electrical charge transfer. Berger relies on a digital-analog multiplication and summation scheme whereby digital tap weights are multiplied with analog charge values in a tapped delay line configuration. However, Berger's device has limited dynamic range.
The most versatile and widest dynamic range FIR filters have been realized using fully digital signal processing techniques. These often require external analog-to-digital (A/D) converters and sometimes have limited sampling rates. For general background, reference is made to Y. Tsividis et al, Design of MOS VLSI Circuits for Telecommunications, Prentice Hall, Englewood Cliffs, N.J., 1985. The digital transversal filter is too slow for many applications.
A third alternative is to realize FIR filters using analog switched-capacitor (SC) techniques. This approach may have speed superior to the digital signal processing techniques and can exhibit a dynamic range superior to the CCD approach. No A/D converters are necessary and the processing is less specialized than required for CCD or BBD filters. The existing programmable SC FIR filter incorporates digital-analog multiplying techniques to permit the control of tap weights by digital means. Y. Lee, et al ("A Switched-Capacitor Realization of Multiple FIR Filters On A Single Chip", IEEE Journal of Solids-State Circuits, Vol. 23, No. 2, pp. 536-542, April 1988) observes that even higher signal processing speed is available in exchange for eliminating such digital programming features in SC FIR filters.
Switched-capacitor programming techniques are well-known in the art. For instance, in U.S. Pat. No. 4,244,030, Alain Albarello teaches a switched-capacitor multiplexing filter. As is universally taught in the art, Albarello uses an operational amplifier as a tap signal summing device at his SC filter output.
In U.S. Pat. No. 4,543,534, Gabor C. Temes, et al teach a switched-capacitor technique for overcoming operational amplifier offset effects in a Multiplying Digital-to-Analog Converter (MDAC). Temes, et al switch capacitive compensation during the non-sampling period to avoid output discontinuities and to compensate for the offset voltage of the summing amplifier. These steps are necessary because all useful transversal tap signal summing techniques known in the art require an operational amplifier.
In U.S. Pat. No. 4,646,258, Saul Miodownik teaches a FIR notch filter that uses a switching arrangement to overcome the effects of component variations on narrow filter notches. Miodownik does not consider monolithic realization of his notch filter nor does he employ simultaneous signal summation but he does use an operational amplifier to sequentially sum his tap signals.
In the above-cited reference, Lee, et al describe a 32-input summing amplifier (FIG. 8A) based on the SC gain amplifier described in above-cited U.S. Pat. No. 4,453,534. Lee, et al give considerable attention to their summing amplifier despite its difficulty because all useful tap signal summing methods known in the art require an operational amplifier.
In U.S. Pat. No. 4,849,662, Douglas R. Holberg, et al disclose a method for time-sharing a digitally-programmable capacitive element suitable for use in a SC filter circuit. Holberg, et al primarily consider a method for conserving chip real estate by reducing the area necessary for filter capacitors but they, too, use an operational amplifier in the tap signal summing portion of their programmable filter despite the difficulty related to the use of such an active device.
In U.S. Pat. No. 4,470,126, Yusaf A. Haque discloses a programmable transversal filter (FIG. 2) that employs an analog cross-point switch to sequentially connect analog signal samples to a bank of multipliers, the outputs of which are summed through an operational amplifier. Haque's multipliers include a switched-capacitor bank in which the capacitance can be selected by means of a binary input signal (FIG. 4). The effective tap weight introduced by Haque's programmable multiplier is equal to a digitally-selected variable capacitance divided by a fixed operational amplifier feedback capacitance. Haque neither teaches nor considers a self-normalizing switched-capacitor multiplying and adding scheme that can be implemented without operational amplifiers.
The monolithic transversal filter realizations known in the art are limited in dynamic range and speed (sampling frequency). Digital realizations can eliminate the most troublesome dynamic range problems but are limited to relatively low operating frequencies. High-speed analog monolithic transversal filter realizations may offer better operating frequencies but are often seriously limited in dynamic range. The monolithic SC transversal filter offers the best compromise in dynamic range and operating frequency but improvement is hampered by the slew-rate limitations of the operational amplifiers used in the multiplier and summation elements of the filter.
Accordingly, a technique without operational amplifiers for multiplication and summation in a SC transversal filter represents an extraordinarily useful improvement to the filters known in the art. However, because of the universal presumption that tap signal summation and multiplication require operational amplifiers, no such techniques have before been suggested or proposed by practitioners in the art. The related limiting effects are clearly felt in the art and are solved by this invention in the manner described below.